Few days ago I got one of my Linux RAID1 arrays go bad. One of the disks got bad sectors and the other one lost it's superblock. So the array was degradated and the only one "good" disk was with bad sectors. I added a new disk and tried to sync the data but it stucked on 36%. Using tools like "dd" or "ddrescue" didn't help neither. The "dd" just kept stopping and the "ddrescue" was recovering with 364Bytes/second so on 3TB disk this was kind of slow. After 2 days I just gave up recovering with this speed. So back to google I found that mdadm is smart enough so when creating new array it preservs the old data. So I decided to give it a try. Still just in case I didn't wanted to loose my data so did a clone of the disk with the "ddrescue" tool.